Currently, image display devices of various methods have been proposed. One of the image display method used in such image display devices is a projection display method in which a modulated light source light is projected onto a screen to display an image. Conventionally, the light source employed in the image display device using such method is a lamp light source. However, the lamp light source has problems in its short lifetime, its limited color reproduction region, and low light usage efficiency.
In order to solve the problems in a lamp light source, a laser light source has been recently tested for use as a light source of the projection display. In the present application, an image display device using a laser light source as a light source is referred to as the laser image display. A laser light source, which is used in the laser image display, has longer lifetime than a lamp light source, and is easy to enhance the light usage efficiency for the sake of its strong directivity of laser light. Furthermore, laser light emitted from a laser light source excels in monochromaticity. It enlarges the color reproduction region compared to that by the lamp light source and enables displaying a vivid color image.
However, the laser image display has a problem of speckle noise. Speckle noise is a noise due to high coherency of laser light, which is used for displaying an image. Such problem did not exist in an image display device in which a lamp light source is used. When the laser light having high coherency is scattered on the screen and reaches an observer, the scattered laser light interfere with each other and the observer recognizes microscopic uneven noise. The speckle noise appears as granular noise of a size defined from an F value (F number) of an eye of the observer and a wavelength of the laser light source and is arrayed at random. When the observer viewing an image on the image display screen, the speckle noise inhibits the observer from recognizing the image on the image display screen, and causes serious image degradation. For reducing the speckle noise, various techniques being related to the laser image display method and the laser image display have been proposed.
Patent Document 1 (JP 06-208089 A) discloses a display device equipped with a diffusion element which performs rotational movement. The laser light passed through the diffusion element which performs the rotational movement is used for the illumination of a modulation element in this display device. The diffusion element which performs the rotational movement, in effect, temporally changes an incident angle of the illumination light with respect to the modulation element. Therefore, an incident angle of the laser light with respect to the image display screen also temporally changes, and hence, the speckle pattern generated on the image display screen temporally changes. Consequently, the speckle noise which the observer recognizes is reduced since the observer observes an image in which the speckle pattern is variedly changing and temporally averaged.
Patent Document 2 (JP 10-293268 A) discloses a laser display device equipped with an optical deflector and a fly eye lens. Laser light emitted from a light source enters through the optical deflector the fly eye lens in such laser display device. The fly eye lens forms secondary light sources of the same number as the elements configuring the same. The laser light having exit the secondary light source array enters a spatial modulator, at which an image is formed, and the image is displayed on a screen. Therefore, in this display device, the light source image which illuminates the spatial modulator is a substantially point light source array. It is difficult to sufficiently remove the speckle noise when the light source which forms its light source image like that above mentioned is used as light source.
Patent Document 3 (JP 2003-98476 A) discloses a laser projection type display system equipped with a diffuser and a fly eye (eyes of a fly) integrator, and also proposes an optimum arrangement of the movable diffuser and the fly eye integrator.
Patent Document 4 (JP 2004-144936 A) discloses an image display device in which a diffusion element is arranged between a light source and an illuminated subject body. In such image display device, reduction in speckle noise is achieved by vibrating the diffusion element by means of a vibration applying means connected to the diffusion element. This document also proposes arranging another diffusion means other than the diffusion element.
As mentioned above, it is possible to reduce the speckle noise by moving the diffusion element such as diffuser and the diffuser plate and changing the angle of the illumination light. However, a precise control of the illumination light angle is practically impossible to perform in such configuration. The device having such configuration has a problem in light usage efficiency of the projection optical system for projecting the light emitted from the modulation element on the image display screen.
In the field of the image display device using a laser light source such as a semiconductor laser, a proposal relating to enhancement in light usage efficiency in which a projection optical system with enhanced light usage efficiency is suggested has seldom been proposed.
In the conventional proposal related to the optical system of the display (laser image display) which employs a laser light source, reduction of the speckle noise is centrally focused. Proposals from a standpoint of enhancement of the light usage efficiency in the entire optical system of the device, which includes the projection optical system, are hardly seen in such conventional proposals. A proposal of a technique which achieves both of the two points at a high level, that is, the sufficient speckle noise reducing effect and the ensuring of high light usage efficiency, has not been yet found.    Patent Document 1: JP 06-208089 A    Patent Document 2: JP 10-293268 A    Patent Document 3: JP 2003-098476 A    Patent Document 4: JP 2004-144936 A